1. Field of the Invention
The present invention relates to a safety controller preferable for construction of a safety system (safety operation system) in a factory, particularly to a safety controller set between various safety switches and production equipment which may be a dangerous source to execute the control for avoiding a state in which a worker is exposed to danger.
2. Description of the Prior Art
To construct a safety system in a factory, various safety controllers are used. The basic function of this type of safety controllers executes the control for stopping production equipment by determining a state in which a worker is exposed to danger between various types of safety switches (e.g. emergency stop switch, two-hand-control switch, mat switch, safety limit switch, light curtain, and electromagnetically-locking safety door switch) and production equipment (robot arm, machine tool, and conveyer).
A safety PCL (Programmable Logic Controller) and a safety relay unit are known as conventional safety controllers. The safety PLC is a controller whose operation reliability is improved compared to a normal PLC by duplexing hardware and software or adding a troubleshooting function. The safety relay unit is a unit having a built-in exclusive safety relay set between a safety switch and production equipment. A safety relay unit is conventionally known which can be applied to various types of safety switches by automatically recognizing the type of a connected safety switch and changing a program to a corresponding operation program (for example, refer to Patent Document 1).
Patent Document 1 is National Publication of International Patent Application No. 2001-521669.
However, because the safety PLC is basically a PLC, it is necessary for a vendor who is a user or an end user to prepare an operation program for prescribing the relation between a signal input from a safety switch and a signal output to a dangerous source. Therefore, the above mentioned is troublesome and a programming error easily occurs. Moreover, the above mentioned is very troublesome because it is necessary to obtain an approval on whether the safety standard is satisfied whenever installation of an operation program in the safety PLC is completed when installing the program in equipment to be exported to Europe which requests a severe safety standard. Furthermore, when the number of safety switches increases and addition of operation programs is necessary because of expansion of equipment in a production line, it is necessary that the added operation programs are newly approved by the safety standard. Therefore, the same problem as described above occurs.
In the case of the safety relay unit, an operation for prescribing the relation between a signal input from a safety switch and a signal output to a dangerous source is fixed in hardware or software. Therefore, if the unit is approved by the safety standard when the unit is shipped, it is not necessary that the unit is repeatedly approved by the standard when installation of the unit in an actual safety system is completed. However, because an exclusive unit is necessary every type of safety switch, it is necessary to purchase an exclusive safety relay unit every type of switch in order to correspond to a safety system including various types of safety switches. Thus, ordering is troublesome and stock management requires a lot of time and the cost will be increased. Moreover, also whenever expanding equipment in a production line, it is necessary to purchase a relay unit corresponding to a newly added safety. Therefore, the cost will be increased.
Because the safety relay unit having a built-in CPU and being able to be applied to various types of safety switches described in Patent Document 1 can be applied to only one safety switch, a plurality of safety switches respectively having a built-in CPU is necessary in order to correspond to a safety system including a plurality of safety switches and therefore, the cost will be extremely increased.